1. Field of the Invention
The invention relates to the repair of hydroelectric assemblies and, more particularly, relates to methods for maintaining stability of assembly components during the repair process.
2. Description of Related Art
Historically, hydroelectric facilities have been utilized for purposes of generating electricity through the use of power resulting from movement of water through gravitational forces. Such facilities can comprise one or more electrical generator units, with each unit powered by a hydraulic turbine mechanism.
Modern hydroelectric facilities typically are designed around a vertically mounted shaft. Attached to the upper portion of the shaft is a generator rotor. Correspondingly, a hydraulic turbine assembly is typically attached adjacent the lower portion of the shaft, and comprises a series of turbine blades. The water enters the area of the turbine mechanism at a point above the turbine blades. Through gravitational forces, the movement of the water causes the rotation of the turbine blades at a speed sufficient so as to cause the generator portion of the facility to appropriately generate electricity.
The internal environment of the hydroelectric turbine assemblies is relatively severe. That is, the turbine blades are subjected to relatively large stresses resulting from the water movement and blade rotation. In addition, the walls surrounding the turbine blades, typically characterized as the liner wall, are also subjected to severe stresses.
Such stresses are commonly explained in accordance with known principles of fluid mechanics. For example, the water flow within a hydraulic turbine will cause a phenomenon known as "cavitation." This phenomenon will subject fluid flow surfaces (e.g. liner walls and turbine blade surfaces) to intense local stressing, which appears to damage flow surfaces by fatigue. Cavitation within a hydraulic turbine will result in pitting and general surface deterioration of liner walls and blades.
The principles of cavitation and other fluid mechanics stress phenomena are relatively well-known, and are explained in conventional texts such as Streeter, Fluid Mechanics (McGraw-Hill 1966, 4th Ed.). Cavitation occurs in a flowing liquid whenever the local pressure of the liquid falls to the liquid vapor pressure. When this point is reached, local vaporization of the liquid will result, causing a hole or cavity in the flow of the liquid. The cavity contains a swirling mass of droplets and vapor. When the pressure exerted on the flowing liquid is raised above the vapor pressure of the liquid, the low-pressure cavity rapidly collapses and the surrounding liquid rushes in to fill the void. At the point of disappearance of the cavity, the inrushing liquid comes together and momentarily raises the local pressure within the liquid to a very high level. When the point of collapse of the cavity is in contact with a metal surface, the surface may be stressed locally beyond its elastic limit, resulting eventually in fatigue, pitting and destruction of the material. In a hydroelectric facility, cavitation occurs on the turbine blades and the interior walls or lining of the turbine in the area of the turbine blades. Typically, cavitation erosion on the surfaces in a hydroelectric facility must be repaired relatively frequently, e.g. once per year.
A common method for repairing cavitation erosion on turbine wall surfaces is to remove the heavily-pitted material by various grinding means, and then replace this removed material by a welding process. In the past, such repair has been accomplished by various hand-held grinding tools and replacing the ground material by welding stainless steel to the ground surfaces. These conventional methods of repair are extremely slow and expensive due to the lengthy down-time of the hydroelectric unit being serviced.
An attempt to speed up this repair process is disclosed in the U.S. Pat. No. 3,793,698 issued Feb. 26, 1974 to Goings. The Goings '698 patent is incorporated herein by reference. The Goings '698 patent discloses a semi-automatic method and apparatus for machining and welding the liner of a hydroelectric structure. The Goings apparatus includes machining tooling installed on a lower portion of the hydroelectric shaft, such that the tooling is brought into operative engagement with the tube wall or liner.
The generator portion of the hydroelectric structure disclosed in the Goings '698 patent comprises a rotor which is moved by the runner or rotor of a hydraulic turbine. Gates control the entry of water into the draft tube of the turbine, with the blades of the turbine runner positioned within the upper cylindrical portion of the draft tube. The rotational axis of the turbine runner is concentric with the rotational axis of the rotor of the generator. The generator rotor has an extending portion which carries an annular shoe adapted to cooperate with an adjacent member which provides for braking the movement of the rotor, in the event of a failure or other emergency.
For purposes of repair, a temporary floor structure is constructed across the draft tube below a runner hub. A collector ring assembly is attached to the lower end of the runner, with the collector ring providing for transference of electrical power and pneumatic lines into the runner.
An external power unit is installed adjacent the extending portion, which rotates with the generator rotor. This power unit can include an electric motor having V-belts which drive a roller or traction member engaging the adjacent face of the annular track carried by the generator rotor portion. A control panel provides for control of the drive motor. Through reduction gearing between the motor and the traction member, energization of the motor results in a rotational force being applied to the generator rotor shaft and, correspondingly, to the runner shaft and runner. During rotation, machining and welding operations can be carried out on the draft tube liner.
In addition to the arrangement for exerting rotational forces on the generator rotor, the Goings arrangement also includes a series of support members temporarily welded to the surface of one of the runner blades. These support members mount a vertical member on which travels a cutting or machining tool holding head. The holding head can be adjustably positioned along the vertical member by means of an adjustment wheel. In addition, the structure also includes a seat for an operator riding the structure and observing the machining operation. A machining tool is adapted to extend from the tool holding head into a machining engagement with the adjacent draft tube wall surface.
In operation, scaffolding is temporarily attached to and supported by the runner blades. The scaffolding is utilized to support welding apparatus, including a welding rod supply reel. The scaffolding structure also carries a vertical member which supports a vertically movable welding head. The welding head is adjustably movable in a vertical direction along the vertical member. A second operator seat is provided for purposes of observing the welding operation. As the turbine runner is rotated, the welding head is made to traverse the curved surface of the tube wall. The arrangement of the scaffolding and the welding equipment is such that a welder can weld pitted areas of the blades as the rotation of the turbine runner proceeds.
After conditioning equipment, such as the cutting tool and the welding head, have been installed, the turbine runner can be rotated by the rotational mechanism previously described. Material of the draft tube wall can be machined off to a desired depth. Weld material, such as stainless steel, can then be applied to the wall. Also, a carbon-steel build-up layer can be initially applied to the wall, and subsequently covered by the stainless steel surface. The tube surface can then subsequently again be machined so as to smooth the stainless steel build-up. Further, severely damaged outer edges of the blades can have sections removed, and the gaps renewed with stainless steel sections. Such operations on the blades are carried out by an operator utilizing hand-held tools, while the machining and welding of the tube surface is performed as the turbine runner is rotated.
While the Goings '698 patent structure has provided advantages in operation over previously-known arrangements for on-site repair of hydroelectric turbine surfaces, several problems still exist with respect to the method of operation of the Goings repair arrangement. For example, with the particular structure utilized in the Goings arrangement, and with the rotational forces applied to the generator rotor, it has been found that rotation of the turbine shaft and the machining tooling mounted to the turbine blades can be relatively jerky and erratic. Accordingly, a relatively severe amount of chatter and vibration can occur during the machining operation.
Further, with the forces exerted directly on the generator rotor relatively far above the turbine blades, the rotational system is far removed from the location of the machining operations. Therefore, the rotational system is not convenient for purposes of precise and spontaneous adjustments of the rotation speed. Still further, with the positioning of the application of rotation forces at the perimeter of the generator rotor, a relatively substantial force is required to achieve appropriate rotational speed. Accordingly, motors of substantial size must be employed. Correspondingly, the motor apparatus for exerting the rotational forces is relatively bulky and expensive. The bulkiness can cause substantial problems and expense, with respect to transportation and assembly of the apparatus at a job site.
A substantial advance in the state-of-art of hydraulic repair systems has been achieved as disclosed in U.S. Pat. No. 4,884,326 issued to Porter et al on Dec. 5, 1989. The Porter et al patent describes a method and apparatus primarily directed to rotation of a hydroelectric assembly for purposes of repairing and resurfacing of turbine liner walls pitted as a result of cavitation. The method is adapted for use in a hydroelectric turbine structure comprising a turbine shaft, generator rotor assembly coupled to an upper portion of the shaft and a turbine blade assembly coupled to a lower portion of the shaft. The turbine blade assembly includes a plurality of turbine blades extending radially from the turbine shaft. A turbine chamber having a substantially cylindrical configuration is formed by a vertically-disposed liner wall adjacent distal ends of the turbine blades.
The method includes removing water from the turbine chamber, and mounting a repair assembly at or substantially adjacent a distal end of at least one of the turbine blades. The repair assembly can include conditioning devices for repairing surface deterioration of the liner wall. A turning apparatus is mounted at or substantially adjacent a distal end of at least one of the turbine blades. The turbine blades and turbine shaft are slowly rotated by exerting forces directly between the turning apparatus and the liner wall, thereby causing the conditioning devices to traverse the liner wall.
The mounting of the turning apparatus includes the mounting of a support assembly directly to one of the turbine blades. A turning wheel is mounted in a pivotable configuration relative to the support assembly. The turning wheel is engaged with the liner wall so as to be in frictional contact therewith. Rotational forces are then exerted on the turning wheel to rotate the turning wheel, thereby causing the turning wheel to traverse the liner wall, and further causing rotation of the turbine blades.
The turning apparatus includes a support structure adapted to be mounted to at least one of the turbine blades. The support structure includes first and second support braces, with each of the braces having one end secured adjacent a distal end of at least one of the turbine blades. A first pivot assembly is then pivotably coupled to an upper end of the support brace and to a distal end of a piston cylinder rod, so that the piston mechanism is pivotable in a pitch mode relative to a horizontal plane extending through the piston cylinder rod. A turning mechanism mounting bracket is also provided, and a second pivot assembly is provided to pivotably couple one end of the piston cylinder with one end of the mounting bracket. A third pivot assembly can pivotably couple another end of the mounting bracket to an upper end of the second support brace.
In accordance with the Porter et al arrangement, the rotation of the turbine blades and turbine shaft by operation of a turning wheel directly against the liner wall provides a requisite "steady" rotational movement for purposes of undertaking repairs of the liner wall. Further, with the position of the turning mechanism adjacent a distal end of one of the blades, the mechanical advantage provided by this positioning is substantial. Accordingly, a relatively small motor drive assembly can be employed for providing the requisite rotation of the turbine blades, notwithstanding the massive size of conventional turbine blades and turbine shafts. Exerting forces between the turbine turning mechanism and the liner wall in the manner as described in the Porter et al arrangement provides a substantial advantage over other arrangements, whereby the turbine blades are rotated through externally-generated forces which must be translated through the turbine shaft.
Various types of machining and grinding mechanisms are relatively well-known and adapted for performing such functions in applications other than hydraulic turbine repair. One type of grinding mechanism suitable for performing hydraulic turbine repair in accordance with the Porter et al arrangement is disclosed in the commonly assigned and co-pending U.S. patent application Ser. No. 397,051, Porter, filed Aug. 22, 1989. The Porter application discloses an arrangement for rotating the turbine blades and turbine shaft to facilitate repair of pitting and general deterioration of the liner walls. A vertical support structure is mounted to one or more of the turbine blades substantially adjacent the wall. The grinding apparatus includes a grinder contact wheel, along with a grinding belt coupled around the wheel. An energizing arrangement is provided for selectively rotating the grinder contact wheel and the belt.
A support arrangement is provided for mounting the grinder contact wheel, grinding belt and energizing arrangement. An adjustable connection mechanism is coupled to the support arrangement and to the vertical support structure for mounting the grinder contact wheel to the support structure. A further connection arrangement is also provided for adjusting the horizontal position of the grinder contact wheel relative to the vertical support structure, with the adjustment being in a radial direction relative to the turbine shaft. The vertical support structure also includes an additional adjustable connection arrangement coupled to the adjustable connection arrangement for adjusting the vertical position of the grinder contact wheel relative to the vertical support structure.
Although the apparatus and methods described in the foregoing paragraphs provide substantial advantages in the state of the art with respect to hydroelectric turbine repair, other aspects of the repair process also require attention. For example, as earlier described, repair of the cavitation damage associated with liner walls and discharge rings requires the removal of pitted or cavitated liner wall material. Typically, this liner wall material is in the form of carbon steel or the like. During the repair process, the liner wall material is replaced by stainless steel weldments.
However, in accordance with known and conventional metallurgical phenomena, welding of the liner wall or discharge ring will result in shrinkage. Typically, the shrinkage factor between carbon steel and stainless steel is in the range of a ratio of approximately 40 to 1. During and subsequent to the welding process, the actual amount of shrinkage which occurs is substantially proportional to the number of welding stringer beads which may be utilized to clad the liner wall. That is, the greater number of stringer beads utilized, the greater will be the shrinkage forces.
Typically, the liner walls are positioned adjacent concrete or other cementitious material positioned behind the liner wall. The liner walls are typically held in place through the use of vertical and horizontal stiffeners. The shrinkage forces resulting from the stainless steel weldments are in part exerted against these vertical and horizontal stiffeners. Again, as the number of stringer beads employed during the welding process increases, the shrinkage forces exerted against the stiffeners correspondingly increase.
Although the shrinkage forces against the stiffeners can present problems, it is the vulnerability of the carbon steel liner wall area between the stiffeners which presents the most significant problems. In these areas, there is no anchoring of the liner walls to the adjacent concrete walls. Significant warpage can thus occur as a result of the shrinkage factor ratio between the carbon and stainless steels.
Efficient operation of hydroelectric turbines requires a substantially optimal "roundness" of the liner wall. If substantial shrinkage or warpage occurs in the liner walls, roundness is destroyed and significant problems can occur with respect to turbine operation. Typical specifications for liner wall roundness often require an "out of round" specification of no more than plus or minus 25/1000 of an inch. The specifications also often require the roundness variation not to exceed plus or minus 10/1000 of an inch between liner wall stiffeners.